1. Field of the Invention
The present invention relates to a photodiode, and a method for manufacturing the same.
2. Description of the Related Art
Recently, in order to cope with a dramatic increase in the capacity of networks, epitomized by the widespread use of the Internet, development of high-speed optical transmission systems has been sought. Today, optical fibers having comparatively large apertures that facilitate alignment, such as plastic optical fibers, multimode glass optical fibers, and the like, are drawing increased attention as optical fibers for use not with trunk line systems but “last one mile” transmission systems, including subscriber line systems. As optical receivers suitable for high speed and large aperture optical fibers, MSM (metal-semiconductor-metal) photodiodes, lateral PIN photodiodes, and the like, in which electrodes are arranged laterally, i.e., in the direction parallel to the surface of the substrate in a finger pattern, are known.
Use of a transparent electrode, instead of an opaque electrode that blocks light, as the finger pattern electrode, i.e., comb electrodes is proposed, in order to further increase the sensitivity of these types of photodiodes. Generally, in a photodiode having a transparent electrode, however, the sensitivity and high-speed response become a trade-off due to (1) a large time constant arising from a high resistance value, and (2) generation of low mobility holes even under the positive electrode if the active layer is an intrinsic or lightly doped N type layer, and the travel distance of the holes becomes long. U.S. Pat. No. 5,777,390 describes a structure for improving the trade-off between the sensitivity and high-speed response. The structure described in the Patent tries to reduce the travel distance of the holes by using a transparent electrode (different in material and clearness from that of the other type of electrode) as the electrode for collecting low mobility holes, i.e., negative electrode, while preventing holes from being generated under the positive electrode by using a low resistance nontransparent electrode as the positive electrode, in order to alleviate the trade-off.
Further, in the structure described in U.S. Pat. No. 5,777,390, use of long transparent electrodes results in high resistance, so that it is difficult to sufficiently alleviate the trade-off between the increased sensitivity and speed. That is, it is difficult to satisfy the demands of increased sensitivity and speed.
Still further, in the aforementioned structure, the transparency of the transparent electrode depends on the material, so that the transmittance is not freely (continuously) controlled. In addition, two different materials capable of satisfactorily contacting the active layer need to be used.